The present invention is generally related to sensors, and more particularly to an improved sensor for measuring soil moisture content and water level.
Apparatus and sensors for providing quantitative measurement of soil moisture and water level are known. Pressure actuated sensors known as "tensiometers" provide an indication of soil moisture content by employing a column of liquid within a closed generally cylindrically shaped container that is separated from the soil by a membrane. Relatively moist soil exerts little or no force on the tensiometer liquid. Relatively dry soil causes a reduction in pressure within the container, i.e., the pressure decreases within the container as the soil develops a greater capacity to soak up the tensiometer liquid. Soil moisture content is determined from the pressure exerted on the tensiometer liquid. However, data from tensiometers can be inaccurate and must be manually gathered and stored.
Relatively accurate electromechanical soil moisture sensors are also known. Soil exhibits a dielectric property which varies in proportion to moisture content. If the dielectric value of a region of soil can be measured, the moisture content of that region of soil can be calculated as a function of the measured dielectric value. Both time domain reflectomotry and capacitance measurement have been employed to determine the dielectric value of a region of soil to calculate moisture content.
In time domain reflectomotry a voltage pulse is transmitted through a length of transmission cable that terminates in the soil. A portion of the pulse is reflected at the cable/soil interface. The magnitude of the reflected portion of the pulse is indicative of the dielectric value of the soil. However, devices that employ time domain reflectomotry are relatively complex to implement and consequently tend to be costly to manufacture.
A capacitance based measurement may also be performed in which a soil "capacitor" is formed between two electrodes that are inserted into the soil. An excitation signal is applied to a circuit such as a Wein Bridge that includes the soil "capacitor" as the only component of unknown value. An output signal is produced in response to the excitation signal, and analysis of the output signal allows calculation of the capacitance value of the soil "capacitor." However, the apparant capacitance between the electrodes is affected by soil salinity which tends to introduce errors into soil moisture calculations when using a soil "capacitor."
Similar sensors are known for measuring water level, such as the level of water in a well. For example, it is known to calculate water level based on pressure. In a column of water, pressure increases as depth increases. Water pressure can be measured with a pressure sensor and employed to calculate water depth. However, variations in atmospheric pressure also affect pressure sensors. To compensate for variations in atmospheric pressure a separate sensor may be employed to measure atmospheric pressure and calculations may be based on differential pressure measurements. However, the equipment required for compensation increases the complexity of the apparatus and also increases the cost to manufacture such devices. It is also known to calculate water level by placing electrodes at opposite ends of a water column and determining the capacitance between the electrodes. However, the apparant capacitance between the electrodes is adversely influenced by the salinity of the water. Further, both types of water level measurement devices must be individually calculated to compensate for variations in construction.